While stepping back from what looked to be a certain defeat in his bid for domestic political support for a strike against Syrian president Bashar Assad's military, Mr Obama still spent most of an address to the nation making the case for action as a deterrent to further use of chemical weapons and a warning to other countries tempted to use them.

"The images from this massacre are sickening. Men, women, and children lying in rows, killed by poison gas, others foaming at mouth, gasping for breath, a father clutching his dead children, imploring them to get up and walk," he said.

But he said he would give the proposal by Syria's ally Russia to stockpile the weapons a try.

"It's too early to tell whether this offer will succeed, and any agreement must verify that the Assad regime keeps its commitments," Mr Obama said. "But this initiative has the potential to remove the threat of chemical weapons without the use of force."

The diplomatic deal that is under discussion, with Assad's agreement, would put Syria's chemical weapons under international control for destruction. Secretary of State John Kerry is meeting Russian Foreign Minister Sergey Lavrov in Geneva to work out details.

Meanwhile Mr Obama said he has ordered the military to remain prepared to carry out attacks if needed. Addressing criticism over his own promise of limited strikes, he said, "Even a limited strike will send a message to Assad that no other nation can deliver."

Mr Obama said: "Our ideals and principles, as well as our national security, are at stake in Syria, along with our leadership of a world where we seek to ensure that the worst weapons will never be used."

One problem is Russian President Vladimir Putin's demand that the United States agree not to use military force in the future. Mr Obama said he would not do that. "If diplomacy now fails and the United States fails to act, the Assad regime will see no reason to stop using chemical weapons," he said. Over time, he added, the weapons could threaten US troops as well as allies in the region.

"America is not the world's policeman," he said. "Terrible things happen across the globe, and it is beyond our means to right every wrong. But when, with modest effort and risk, we can stop children from being gassed to death and thereby make our own children safer over the long run, I believe we should act."